Our continuing attempt to clarify the regulatory mechanisms in hepatic regeneration involves not only a quest for humoral agents, but also for the growth-related intracellular changes that they generate. To evaluate the role of factors in the portal blood we are studying DNA replication (as an index of cell proliferation) in rat livers following removal of portal splanchnic viscera and 68% hepatectomy, the animals being maintained by continuous intravenous alimentation plus insulin. Regeneration does occur; our next step will be to examine in detail the roles of insulin and glucagon, and subsequently other hormones as well. In the liver remnant we are investigating microtubule proteins (tubulins), which at the time of mitosis form the fibers of the mitotic spindle. Tubulins are present in both the soluble and particulate fractions of liver homogenates. Both amount and rate of synthesis of "soluble" tubulins increase during early regeneration. We will examine particulate tubulins and investigate interrelations of tubulin synthesis to that of other macromolecules (DNA and RNA) during initiation of the growth process.